1. Field of the Invention
The present invention relates to a switching regulator for supplying power from the power supply to the load side via a switching element, with which a predetermined voltage at the load side (output setting voltage) can be acquired by switching this switching element under a predetermined control, and more particularly to a switching regulator that has a soft start function.
2. Description of the Related Art
As one system of acquiring output setting voltage, a switching regulator system, which switches a switching element under a predetermined control, has been conventionally used since it is compact and can implement high efficiency. In order to control rush current which flows into the switching element when power is turned ON, a soft start operation is executed (e.g. Japanese Patent Application Laid-On No. 7-336999).
FIG. 6 is a circuit diagram of a conventional switching regulator. In the switching regulator 101, the switching element 114, which is a PMOS transistor, supplies power from the power supply side (VCC) to the load side, and holds the load side output (VOUT) at the output setting voltage by switching under a predetermined control, that is, according to the output of the comparator 110, which is described later. To the load side of the switching element 114, a smoothing circuit 115, which comprises a coil, capacitor and diode, is connected for smoothing the voltage from the switching element 114. The load side voltage (VOUT) is divided by the voltage divider 116, which is composed of a serial resistor, and is input to the inversion input side of the error amplifier 111. Between the output (FB) and the inversion input side of the error amplifier 111, an oscillation stop circuit 117, which is composed of a resistor and capacitor, is connected. The error amplifier 111 amplifies the error between the error comparison reference voltage 118 and the voltage which is input to the inversion input side, and outputs the amplified error. The output of the error amplifier 111 (FB) and the output of the later mentioned soft start circuit 113 (SS) are input to the comparator 110, and the lower voltage thereof and the triangular wave voltage from the triangular wave generator (TRI) 112 are compared. The output of the comparator 110 is inverted by the inversion buffer 119, and is input to the gate of the above mentioned switching element 114. In this way, the switching element 114 is controlled by the feedback circuit from the load side output (VOUT).
The above mentioned soft start circuit 113 comprises a constant current source 122 and a capacitor 123 for generating voltage which gradually rises when power is turned ON, and further comprises a low power supply voltage malfunction prevention circuit (UVLO) 120 and a thermal shutdown circuit (TSD) 121.
The reference voltage generator (VREF) 124 is also installed, and the output thereof (Vref) is the power supply for the error amplifier 111, triangular wave generator 112 and soft start circuit 113.
FIG. 7 is a characteristics diagram showing the voltage values in transition from the time of power ON to normal operation. When power is turned ON, the voltage of the output of the soft start circuit 113 (SS) is lower than that of the output of the error amplifier 111 (FB). Therefore in the comparator 110, the output of the soft start circuit 113 (SS) and the triangular wave are compared. If the output of the soft start circuit 113 (SS) is higher than the triangular wave, high level is output from the comparator 110, which is inverted by the inversion buffer 119, and low level is input to the gate of the switching element 114. As a result, the switching element 114 closes (turns ON). If the output of the soft start circuit 113 (SS) is lower than the triangular wave, low level is output from the comparator 110, which is inverted by the inversion buffer 119, and high level is input to the gate of the switching element 114. As a result, the switching element 114 opens (turns OFF).
And in the beginning of power ON, the period when low level is input to the gate of the switching element 114 is short and the high level period is long. And by gradually increasing the low level period and decreasing the high level period, the rush current to flow into the switching element 114 is controlled.
When normal operation starts, the voltage of the output of the error amplifier 111 (FB) becomes lower than that of the output of the soft start circuit 113 (SS). Therefore in the comparator 110, the output of the error amplifier 111 (FB) and the triangular wave are compared.
If the output of the error amplifier 111 (FB) is higher than the triangular wave, the high level is output from the comparator 110, which is inverted by the inversion buffer 119, and low level is input to the gate of the switching element 114. If the output of the error amplifier 111 (FB) is lower than the triangular wave, low level is output from the comparator 110, which is inverted by the inversion buffer 119, and high level is input to the gate of the switching element 114.